Field of the Invention
The present invention relates to a control technology of a vibration type actuator using an electro-mechanical energy conversion element (“piezoelectric element” hereinafter).
Description of the Related Art
The vibration type actuator includes a vibrator in which a vibration, such as an elliptical motion, is excited when a drive signal as a two-phase frequency signal having a phase difference is applied to the piezoelectric element, and moves a moving body among the vibrator and a contact body that contacts the vibrator (moves the vibrator and the contact body relative to each other). A method for controlling driving of this vibration type actuator (moving of the moving body) includes a frequency control that changes a frequency of the two-phase drive signal and a phase difference control that changes a phase difference of the two-phase drive signal. The frequency control is likely to control high-speed driving, and the phase difference control can provide a lower-speed control than the frequency control. A phase difference range near 0° or 180° in the phase difference control is a so-called dead zone in which an unstable drive state reduces a generated drive force and finally stops driving. The influence of the dead zone can lower the controllability of the vibration type actuator.
In order to reduce the influence of the dead zone, Japanese Patent Laid-Open No. 2010-166736 discloses a control method for changing a frequency without changing a phase difference in the low-speed driving that causes an unstable speed in the phase difference control.
However, even if the frequency control is performed in the dead zone of the phase difference control, it is difficult for the frequency control to provide low-speed driving as well as the phase difference control and to perform desired low-speed driving. In addition, since a relationship (characteristic) between the phase difference and the speed of the vibration type actuator has an individual difference, a phase difference control needs to fit each individual vibration type actuator.